In the background art, an NOx occlusion reduction type catalyst is known as a catalyst which reduces and purifies a nitrogen compound (NOx) in an exhaust gas discharged from an internal combustion engine. When the exhaust gas is under a lean atmosphere, the NOx occlusion reduction type catalyst occludes the NOx contained in the exhaust gas. When the exhaust gas is under a rich atmosphere, the NOx occlusion reduction type catalyst detoxifies the occluded NOx through reducing and purifying by hydrocarbon contained in the exhaust gas, and discharges the NOx.
In the NOx occlusion reduction type catalyst, a sulfur oxide contained in the exhaust gas (hereinafter, referred to as SOx) is also occluded. There is a problem that when the SOx occlusion amount increases, the NOx purification capacity of the NOx occlusion reduction type catalyst is reduced. For this reason, in a case where an SOx occlusion amount reaches a predetermined amount, in order that the SOx is desorbed from the NOx occlusion reduction type catalyst to recover the NOx occlusion reduction type catalyst from S-poisoning, it is necessary to regularly perform the so-called SOx purge in which an unburned fuel is supplied to an upstream-side oxidation catalyst by the post injection or the exhaust pipe injection to raise an exhaust temperature to an SOx desorption temperature (for example, see Patent Literature 1).